Recent clinical trials in obese subjects have found that low-carbohydrate diets result in greater weight loss compared with low-fat diets on a time scale of months when diet adherence was likely the highest. One hypothesis regarding the mechanism of improved weight loss with low carbohydrate diets is that such diets significantly modify the bodys hormonal milieu to influence metabolic regulation and energy expenditure. Low-carbohydrate diets may thereby offer a metabolic advantage over low fat diets. In addition, low-carbohydrate diets may also decrease hunger or increase satiety compared to low-fat diets. Determining the mechanism whereby one diet leads to greater weight loss than another is hampered by the inability to accurately measure food intake or physical activity in an outpatient setting. We investigate how human research subjects respond to different diets while they stay as inpatients for extended time periods on the metabolic ward at the NIH Clinical Center. Our studies are designed to investigate how the human body responds to carefully controlled diet and physical activity changes by through alterations of metabolism, body weight, and body composition.